Gentry Sorenson – Field Agronomist
Understanding current levels of corn rootworm is important to understand in each corn field. Corn rootworm adults emerge from mid-July through mid-August. Male corn rootworms emerge before females, and northern corn rootworms emerge after western corn rootworm, usually around five to seven days after. The two types of corn rootworm beetles that are more often seen in Iowa corn fields are northern rootworms, which are light green to dark green and a quarter inch long. The second is the western corn rootworm, which is yellow with three dark stripes that run lengthwise.
Before adult beetle emergence, corn rootworm larvae feed on corn roots and may cause economic loss. It is important to evaluate corn roots in fields and across the years a field is corn. The evaluation of corn roots helps farmers understand if the current management techniques in each field are effective against this pest. A severe corn rootworm infestation can destroy nodes four through six of the corn plant. These nodes are closest to the soil surface, and when those nodes are pruned, it interferes with water and nutrient uptake. Plants can also become unstable, which can interfere with harvest because plants are lodged and not standing well in the field. This can cause difficulty and slow harvest conditions. When nodes are pruned by corn rootworm, research has suggested that there is a 15 percent yield loss for every node that has been pruned back one and a half inches from the stalk.
Inspect your corn roots after you begin to see beetles in your fields. Start by using a spade to dig up corn plants. Use a garden hose with a sprayer to carefully wash the corn roots to remove all dirt. Look at the roots to understand the level of injury that you see on the roots that you are evaluating. The node injury scale available online is a reference that is available to help score damage from corn rootworms on the roots that you are scoring. Videos have also been created to help with understanding the process of digging roots.
Another method to assess root injury includes setting up sticky traps to catch corn rootworm. Sticky traps are used to understand the number of adults that are present in the field. The traps are set up in transects across the field. When 2 beetles or more are captured per day, consider using a different management tactic for corn rootworm control the next time the field is in corn. Resources for corn rootworm or additional crop concerns can be found at https://crops.extension.iastate.edu.